The present invention relates to refrigeration systems and more specifically to an improved multi-stage refrigeration system particularly suited to supermarket refrigeration.
A typical supermarket has many different refrigerated display cases containing various food items held at different temperatures. Normally, these display cases each contain their own refrigerant evaporator, which is connected via refrigerant lines to remotely located condensing equipment (compressor and condensers). Some smaller convenience stores have only a few display cases, which in some instances have the entire condensing apparatus contained within the confines of the display case itself. While this arrangement may be satisfactory for a small store, a larger supermarket with many display cases would suffer from noise, wasted space, and disruptive maintenance if it was so equipped.
The present invention resides in the provision of a system which combines some of the more advantageous features of both such known systems and adds a whole new dimension, for significantly improved performance, efficiency and economy. The present invention also incorporates a unique, very simple, reliable lubrication system ideally suited for the refrigerating system of the present invention.
In an exemplary embodiment of the system of the present invention each display case contains a low horsepower full hermetic booster compressor and a suction/liquid heat exchanger, in addition to the usual evaporator. Each booster compressor thus operates with a relatively constant evaporating pressure, as well as with a low and relatively constant discharge pressure. Furthermore, each compressor is modulated or cycled to control only its own case temperature. Each booster compressor can be small, very quiet, and is located in an enclosed compartment within the display case. In most applications they will require no cooling airflow whatsoever.
The condensing apparatus remains remotely located and, in turn, modulates in order to maintain a controlled pressure sink for the refrigerant vapor received from all of the display case boosters. The condensing apparatus also preferably incorporates a refrigerant subcooler which serves to generate a cool pressurized liquid source for the display cases. The refrigerant vapor generated by the subcooler is blended with the vapor returning from the display case boosters prior to entering the inlet of the condensing apparatus compressors. The system has much in common with the multi-stage system disclosed in applicant's above-identified parent applications (the disclosures of which are herein incorporated by reference), with the significant difference being that the present low-stage compressors are the booster compressors disposed in the display cases, remote from the condensing equipment location and the high-stage compressors.
With the present invention, the vapor lines leaving the display cases will be smaller than usual and will be warmer than internal store ambient temperatures. Thus, no insulation will be required as it is desirable that these vapor lines lose some of their superheat enroute to the condensing apparatus compressors. This interstage heat rejection significantly enhances the overall efficiency of the system. Furthermore, refrigerant R-22 can be utilized as the sole refrigerant from the overall system with no danger of overheating because the high-stage compressors will receive vapor at pressures and temperatures comparable to that received by air conditiioning compressors. This is, of course, desirable because R-22 is not an ozone depleting refrigerant and because it has ideal characteristics for use in the present system (i.e., relatively low density and high latent heat).
The system of the present invention always operates at a high efficiency level (with attendant minimum operating cost) and gives the additional benefit of precise control of each individual display cases temperature, with no efficiency penalty associated with that individual control. High system efficiency results from the following:
1. All system compressors always operate at moderate pressure ratios, thus allowing operation at improved overall efficiency levels.
2. Mechanical liquid subcooling economies are always present and require only an additional heat exchanger within the condensing apparatus.
3. Liquid/suction heat exchange takes place effectively within the confines of the display case, with no overheating danger because the entering liquid temperature is controlled.
4. Low-stage booster compressor discharge vapor cooling occurs freely by exposure to store ambient conditions, which significantly enhances system efficiency by reducing the volume of vapor to be compressed by the high-stage compressors.
5. No deliberate efficiency ribbing pressure drop between evaporator pressure and compressor suction pressure is required for control purposes such as is the case with current systems equipped with evaporator pressure regulators.
6. Condensing pressures are allowed to fall as the outdoor ambient temperature falls, and sufficient subcooling for liquid feed purposes is always maintained by the subcooling heat exchanger, even under low ambient temperatures when high-stage compressor operation may be terminated.
Additional factors which are very important in the selection of a supermarket refrigeration system include reliability, noise, and cost. As will become apparent, the system of the present invention offers significant benefits in each of these areas as well.
Additional objects, advantages and features of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings.